All planets, minor planets and comets orbit the sun in an ellipse.
- All elliptical orbits consist of two foci.
- The Sun is positioned on one of the foci, not in the centre.
- All planets orbit the sun in an ellipse, but as they are almost approximately circular, we would simplify their orbit into a circle.
The Earth has an elliptical orbit as well.
- It comes closest to the Sun at 147 million km, and furthest from the Sun at 152 million km.
- Its average orbital distance is 150 million km, which is also called 1 Astronomical Unit (1 AU).
  - The Astronomical Unit (AU) is a measure of the distance based on the average distance between the Earth and the Sun.
The distance the planet is from the Sun affects the average surface temperature of the planet.
- Venus is an exceptional case for this data set. This is because Venus has a thick atmosphere of carbon dioxide that causes a runaway greenhouse effect which prevents heat from escaping. Hence, Venus is hotter than Mercury even though it is further away from the Sun.
- As for the rest of the planets, the closer it is to the Sun, the hotter it becomes.
- The Earth is the only planet in the solar system that has the right temperature and conditions for life. The other planets are either too hot or too cold.
The densities of the planets can be classified into two distinct groups.
- The 4 closest planets to the Sun have high densities, while the furthest 4 have lower densities.
- Mercury, Venus, Earth and Mars are mainly made of dense, heavy materials (metals, rocks), which explains their high densities.
- Jupiter, Saturn, Uranus and Neptune are mainly made of lighter, less dense materials (ices, water and gases) which contribute to their low densities.
The orbital periods of the planets are visibly affected by their distances from the Sun.

The closer the planet, the faster the orbital period, and vice versa: the further the planet, the longer the orbital period.
The gravitational field strength of a planet is directly proportional to the mass of the planet.
- This means, the more massive the planet, the higher the gravitational field strength.

The gravitational field strength of a planet is inversely proportional to the distance between the planet and the test object.
- The further the test object from the planet, the weaker the gravitational field strength.
The 8 major planets’ gravitational field strength on their surfaces can be summarised in this table here:
- As seen here, Jupiter has the highest gravitational field strength amongst the planets, and it is roughly 2.5 times stronger than the Earth’s gravitational field strength.

6.03 - The Solar System (II)

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